Microsensors are utilized to detect a variety of gaseous and vaporous species in circumstances where the physical size of the sensor is a constraint. Such uses can include detection of species within the human body, such as monitoring the glucose level in patients afflicted with diabetes mellitus by sensing the concentration of oxygen in their blood. Other uses include in vivo or in vitro blood gas sensing, monitoring gas concentrations in closed-loop life support systems, and anesthetic gas monitoring.
One type of sensor currently in use is a so-called planar Clark-type sensor. In such sensors a sensing electrode and a reference electrode are deposited on a substrate using conventional semiconductor fabrication techniques. A hydrogel layer is then deposited on the substrate and serves as the electrolytic medium in which the species to be detected dissolves, facilitating its detection by the sensing electrode.
Madou et al. in U.S. Pat. No. 4,812,221 discloses a sensor for gaseous and vaporous species which comprises a substrate with passages connecting the top and bottom surfaces of the substrate, and through which gas can flow from one surface of the substrate to the other. A gas and vapor permeable sensing electrode is positioned adjacent to the passages. An electrolytic medium is then placed in contact with the top surfaces of the electrode and the substrate. Means for encapsulating the electrode and the electrolyte can be utilized, if desired, to seal the components into one unit and maintain the operational characteristics of the sensor over time.
The above steps provide a microsensor whose active detecting region is limited to that part of the gas permeable electrode which is in contact with the substrate passages or can be reached through diffusion by the gaseous species. This affects its response time and sensitivity because the species must pass through the passages and come into contact with the electrode and electrolytic medium in order to be detected. The number of points at which the gas, electrode, and electrolyte meet determines the amount of interactions necessary to generate a detection (sensitivity).
It is therefore desirable to have a microsensor having faster response time and a higher sensitivity owing to a more distributed active sensing region, so that gaseous or vaporous species can be detected and analyzed within the timescale necessary for their use in time-constrained applications.